The inventor has recognized that after engine shutdown, residual engine heat is sufficient to cause fuel on wetted surfaces of the injectors to decompose and deposit coke.
Coke buildup leads to narrowing of the fuel injector orifices. Without adequate maintenance coking results in uneven burning and eventually can incapacitate the engine. Fuel injectors are often located in relatively inaccessible interior portions of the engine. Extending the service life of an injector can result in significant maintenance cost savings and reduced engine downtime.
Especially critical are small orifices in injectors positioned towards the upper side of an engine, since rising hot gases accumulate and heat the upper engine area significantly for a long period after engine shutdown. The inventor observed that coke buildup occurred to a much greater degree in the upper areas of the engine and consequently recognized the cause was hot gas accumulation after engine shutdown at temperatures sufficient to result in fuel decomposition and subsequent coke accumulation.
This problem has not been generally recognized in the prior art, and specifically the effect of hot gas accumulation in the upper areas of the engine after shutdown has not been appreciated. Several injector purging systems are present in the prior art, however, these systems provide for air purging of fuel from the injectors and scavenging of fuel from fuel manifolds on engine shutdown.
The prior art does not appear to contemplate the state of fuel wetted surfaces after shutdown. Possibly it has been presumed that any fuel will quickly evaporate, with no significant side effect. For example, U.S. Pat. No. 3,541,788 to Schultz provides a system for scavenging fuel from liquid fuel nozzles on shutdown, and U.S. Pat. No. 4,095,418 to Mansson et al provides a system to flush or purge fuel from injectors with compressed air. In both cases a stop valve is activated on shutdown to prevent fuel from escaping into the engine from the injectors. Prior art systems relate only to recovery of fuel from the interior of the injector nozzles. No consideration is given to fuel wetted injector surfaces in communication with hot combustion chamber gases.
Another prior art example is U.S. Pat. No. 3,344,602 to Davies et al which provides a fuel purging system for gas turbine engine injectors which expels residual fuel from the injectors with compressed air. It is specifically noted therein (at Col 2 lines 20-28) that during purging, the decelerating engine compressor and turbine rotors remain rotating to provide sufficient air flow through the engine to discharge the expelled/purged fuel from the combustion zones of the engine.
Recommended operation of a gas turbine engine includes allowing the engine to cool during an idling period before complete engine shutdown. In practice however, pilots especially of small aircraft, often do not allow the engines to idle since cargo access and passenger egress are not safely conducted while the engine idles.
No consideration is given in the prior art to what happens later to fuel wetted injector surfaces. Air flow through the engine diminishes as the engine rotors gradually cease rotating. The metal components of the engine and especially the combustion chamber remain hot for up to one hour after engine shutdown. Heat from these engine areas eventually dissipates through convection into the surrounding metal engine structure and through heat exchange with relatively static air in the engine.
Engine components surrounding the combustion chamber serve as insulation to retain heat and prevent air circulation. Air flow through an operating gas turbine engine is directed axially and all engine structures are designed to minimize resistance to axial air flow. However, when the rotors cease rotating, axial air flow ceases and hot air in the engine rises to the upper portions of the engine, where it is trapped. Hot air is produced on contact with the hot engine components, and together with convection through the metallic engine structure, eventually dissipates the heat from the combustion chamber. During the period immediately after engine shutdown, trapped hot air in the upper portions of the combustion chamber prevents rapid cooling of this portion of the combustion chamber.
The combination of fuel wetted injector surfaces and heat retention in the adjacent combustion chamber results in fuel decomposition and coke buildup. Temperatures of up to 250.degree. F. have been measured after shutdown in the upper areas of the combustion chamber. At this elevated temperature coking can occur.
During engine operation, the passing of fuel through the interior of the injectors and air flow over the exterior of the injectors provide an efficient cooling system for the injectors and coke does not form during operation to any significant extent. However, after engine shutdown, the injectors are not cooled in conventional systems. The trapped hot air accumulating in the upper areas of the combustion chamber is sufficient to heat the injectors to a temperature such that fuel on wetted surfaces decomposes to form a layer of coke. Coke formation is especially detrimental where very narrow orifices are used and coke layers in the order of a few thousandths of an inch can result in measurable decreases in fuel efficiency.